Crop selectivity to specific herbicides can be conferred by engineering genes into crops which encode appropriate herbicide metabolizing enzymes. In some cases these enzymes, and the nucleic acids that encode them, originate in a plant. In other cases, they are derived from other organisms, such as microbes. See, e.g., Padgette et al. (1996) “New weed control opportunities: Development of soybeans with a Round UP Ready™ gene” in Herbicide-Resistant Crops (Duke, ed.), pp 54-84, CRC Press, Boca Raton; and Vasil (1996) “Phosphinothricin-resistant crops” in Herbicide-Resistant Crops (Duke, ed.), pp85-91. Indeed, transgenic plants have been engineered to express a variety of herbicide tolerance/metabolizing genes, from a variety of organisms. For example, acetohydroxy acid synthase, which has been found to make plants that express this enzyme resistant to multiple types of herbicides, has been introduced into a variety of plants (see, e.g., Hattori et al. (1995) Mol Gen Genet 246:419). Other genes that confer tolerance to herbicides include: a gene encoding a chimeric protein of rat cytochrome P4507A1 and yeast NADPH-cytochrome P450 oxidoreductase (Shiota et al. (1994) Plant Physiol. 106:17), genes for glutathione reductase and superoxide dismutase (Aono et al. (1995) Plant Cell Physiol. 36:1687, and genes for various phosphotransferases (Datta et al. (1992) Plant Mol Biol 20:619).
One herbicide which is the subject of much investigation in this regard is N-phosphonomethylglycine, commonly referred to as glyphosate. Glyphosate is the top selling herbicide in the world, with sales projected to reach $5 billion by 2003. It is a broad spectrum herbicide that kills both broadleaf and grass-type plants. A successful mode of commercial level glyphosate resistance in transgenic plants is by introduction of a modified Agrobacterium CP4 5-enolpyruvylshikimate-3-phosphate synthase (hereinafter referred to as EPSP synthase or EPSPS) gene. The transgene is targeted to the chloroplast where it is capable of continuing to synthesize EPSP synthase from phosphoenolpyruvic acid (PEP) and shikimate-3-phosphate in the presence of glyphosate. In contrast, the native EPSP synthase is inhibited by glyphosate. Without the transgene, plants sprayed with glyphosate quickly die due to inhibition of EPSP synthase which halts the downstream pathway needed for aromatic amino acid, hormone, and vitamin biosynthesis. The CP4 glyphosate-resistant soybean transgenic plants are marketed, e.g., by Monsanto under the name “Round UP Ready™.”
In the environment, the predominant mechanism by which glyphosate is degraded is through soil microflora metabolism. The primary metabolite of glyphosate in soil has been identified as aminomethylphosphonic acid (AMPA), which is ultimately converted into ammonia, phosphate and carbon dioxide. The proposed metabolic scheme that describes the degradation of glyphosate in soil through the AMPA pathway is shown in FIG. 8. An alternative metabolic pathway for the breakdown of glyphosate by certain soil bacteria, the sarcosine pathway, occurs via initial cleavage of the C—P bond to give inorganic phosphate and sarcosine, as depicted in FIG. 9.
Another successful herbicide/transgenic crop package is glufosinate (phosphinothricin) and the Liberty Link™ trait marketed, e.g., by Aventis. Glufosinate is also a broad spectrum herbicide. Its target is the glutamate synthase enzyme of the chloroplast. Resistant plants carry the bar gene from Streptomyces hygroscopicus and achieve resistance by the N-acetylation activity of bar, which modifies and detoxifies glufosinate.
An enzyme capable of acetylating the primary amine of AMPA is reported in PCT Application No. WO00/29596. The enzyme was not described as being able to acetylate a compound with a secondary amine (e.g., glyphosate).
While a variety of herbicide resistance strategies are available as noted above, additional approaches would have considerable commercial value. The present invention provides novel polynucleotides and polypeptides for conferring herbicide tolerance, as well as numerous other benefits as will become apparent during review of the disclosure.